Material for Acquisition of Liquids and Disposable Absorbent Article Comprising the Material

ABSTRACT

The present invention relates to a material for acquisition of liquids comprising individualized, crosslinked cellulosic fibers having an effective amount of a polymeric acid crosslinking agent reacted with the fibers in intra-fiber crosslink ester bond form. The material further comprises at least one basic substance, selected from basic polymers. The invention also relates to disposable absorbent articles, such as diapers, containing this material. The material can be used in a method of reducing the electrolyte concentration of aqueous mediums containing electrolytes, such as urine.

FIELD OF THE INVENTION

The present invention relates to a material for acquisition of liquids,absorbent articles containing this material and methods for reducing theelectrolyte concentration of aqueous mediums. The material comprisesindividualized, cellulosic fibers crosslinked with a polymeric acid andthe material comprises at least one basic polymer.

BACKGROUND OF THE INVENTION

Disposable absorbent articles are broadly available and consumers areused to a high performance for the collection and retention of menses(in the case of sanitary napkins or panty liners) or for the collectionand retention of urine and fecal material (in the case of e.g.disposable diapers). However, consumers do not only expect a superiorabsorbency behaviour, but place more and more emphasis on the wearingcomfort of such articles, and namely on the dryness of those articles.Typically, such articles comprise multiple absorbent layers, at leastone layer being primarily designed to store liquid (storage layer), andat least one other layer primarily designed to acquire and/or distributeliquid (acquisition layer). The storage layer may includesuper-absorbent material that is admixed with the traditionally usedpulp fiber material. Such super-absorbent materials may be adapted toabsorb many times (e.g. 10, 20, or 30 times) their own weight, andtherefore be desirable when designing an article of improved fluidhandling properties. Recent absorbent articles may employ higherconcentrations of super-absorbent material than absorbent articles ofthe past, for example concentrations of superabsorbent material inexcess of 50% of the total weight of the storage member. These productsmay achieve a relatively high absorbing capacity with a relatively thinstorage member, thereby potentially reducing the overall thickness ofthe absorbent article product. While super-absorbent materials may becapable of storing substantial amounts of liquid, they may not able todistribute the liquid from the point of impact to more remote liquidstorage areas of the absorbent article as fast as the liquid isdischarged to the article. For this reason acquisition layers maysometimes be included in an absorbent article. Acquisition layers aretypically configured to provide for the interim acquisition of liquidand for the distribution of liquid to various regions of the storagelayer. After the initial acquisition of liquid by the acquisition layer,the liquid may subsequently be absorbed by and finally stored in thestorage layer. Thus, the acquisition layer may provide a desirable wayto maximize the absorbent capacity of the storage layer. An example ofan acquisition layer is disclosed in PCT Publication No. WO 95/34710.

Besides initial acquisition and distribution of liquids, another factorthat may be considered when evaluating the performance of disposableabsorbent articles is the absorbent capacity of the super-absorbentmaterial in the storage layer. Super-absorbent materials may be providedin the form of super-absorbent polymers (SAPs), which are lightlycrosslinked hydrophilic polymers that can absorb up to about one hundredtimes their own weight, or more, of distilled water. One commonly usedSAP for absorbing electrolyte-containing liquids such as urine, ispartially crosslinked, neutralized polyacrylic acid including forexample 50% to 75% or 70% to 100% neutralized carboxyl groups. Onedesirable quality of an SAP in a hygienic article, such as a diaper, mayinclude the ability to retain the absorbed fluid under a confiningpressure. In at least some instances, the swelling and absorbentproperties of SAPs may be attributed to (a) electrostatic repulsionbetween the charges along the polymer chains, and (b) osmotic pressureof the counter ions. However, it is commonly known in the art that theseabsorption properties are reduced in solutions containing electrolytes,such as saline, urine or blood. Thus, SAPs may function less effectivelyin the presence of such physiologic fluids. This decrease in absorptioncapacity is often referred to as “salt poisoning”.

There have been attempts to counteract the salt poisoning effect byremoving salts. For example, see US 2003/0144379 to Mitchell, et al.;PCT Publication No. WO 99/33843 to Garoff, et al.; PCT Publication No.WO98/37149 to Goldman; WO 92/20735 to Tanaka, et al.; EP 0 210 756 A1 toWong; JP 57-35938 A2; JP 11-89878 A2; and JP 01-164436 A2.

Besides salt poisoning, the absorption capacity of SAPs may also bereduced by reducing the degree of neutralization, e.g by acidifyingliquids. U.S. Pat. No. 4,657,537 describes a disposable absorbentarticles having an ion-exchanging topsheet. This topsheet exchanges onlycations against protons. It does not remove anions and it acidifies theliquid by lowering the pH.

It may be desirable to provide material for the acquisition ofelectrolyte containing liquids with good acquisition, distributionand/or absorption properties. It may also be desirable to provide animproved acquisition material based on acid crosslinked cellulosicfibers which reduce the electrolyte concentration of a liquid withoutacidifying the liquid.

SUMMARY OF THE INVENTION

In order to provide a solution to the problems stated above, at leastone embodiment described herein provides a material for the acquisitionof liquids. The material comprises at least one basic polymer. Thematerial further comprises individualized, crosslinked cellulosicfibers. The fibers have an effective amount of at least one acidiccrosslinking agent reacted with the fibers in intra-fiber crosslinkester bond form. The acidic crosslinking agent is a polymer comprising aplurality of acidic functional groups.

Another embodiment described herein provides a material for theacquisition of liquids. The material comprises at least conductivityreducing substance. The material further comprises individualized,crosslinked cellulosic fibers. The fibers have an effective amount of atleast one acidic crosslinking agent reacted with the fibers inintra-fiber crosslink ester bond form. The acidic crosslinking agent isa polymer comprising a plurality of acidic functional groups.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims pointing out anddistinctly claiming the present invention, it is believed the same willbe better understood by the following drawings taken in conjunction withthe accompanying specification wherein like components are given thesame reference number.

FIG. 1 is a top plan view of a disposable diaper, with the upper layerspartially cut away.

FIG. 2 is a cross-sectional view of the disposable diaper shown in FIG.1

DETAILED DESCRIPTION OF THE INVENTION

All percentages, parts and ratios are based upon the total weight of thecompositions of the present invention, unless otherwise specified. Allsuch weights as they pertain to listed ingredients are based on theactive level and therefore do not include solvents or by-products thatmay be included in commercially available materials, unless otherwisespecified. All molecular weights as used herein are weight averagemolecular weights expressed as grams/mole, unless otherwise specified.

DEFINITIONS

“Material for acquisition of liquids” and “liquid acquisition material”are used herein interchangeably.

“Absorbent article” refers to devices that absorb and contain liquid,and more specifically, refers to devices that may be placed against orin proximity to the body of the wearer to absorb and contain the variousexudates discharged from the body. Absorbent articles include but arenot limited to diapers, adult incontinent briefs, training pants, diaperholders and liners, sanitary napkins and the like. Absorbent articlesalso include wipes, such as household cleaning wipes, baby wipes, andthe like

“Disposable” is used herein to describe articles that are generally notintended to be laundered or otherwise restored or reused i.e., they areintended to be discarded after a single use and, possibly, to berecycled, composted or otherwise disposed of in an environmentallycompatible manner.

“Disposed” is used to mean that an element(s) is formed (joined andpositioned) in a particular place or position as a unitary structurewith other elements or as a separate element joined to another element.

“Diaper” refers to an absorbent article generally worn by infants andincontinent persons about the lower torso.

The terms “thickness” and “caliper” are used herein interchangeably.

“Attached” or “Joined” encompasses configurations whereby an element isdirectly secured to another element by affixing the element directly tothe other element, and configurations whereby an element is indirectlysecured to another element by affixing the element to intermediatemember(s) which in turn are affixed to the other element.

“Comprise,” “comprising,” and “comprises” is an open ended term thatspecifies the presence of what follows e.g. a component but does notpreclude the presence of other features, elements, steps or componentsknown in the art, or disclosed herein.

The term “hydrophilic” describes fibers or surfaces of fibers, which arewettable by aqueous fluids (e.g. aqueous body fluids) deposited on thesefibers. Hydrophilicity and wettability are typically defined in terms ofcontact angle and the strike through time for the fluids, for examplethrough a nonwoven fabric. This is discussed in detail in the AmericanChemical Society publication entitled “Contact angle, wettability andadhesion”, edited by Robert F. Gould (Copyright 1964). A fiber orsurface of a fiber is said to be wetted by a fluid (i.e. hydrophilic)when either the contact angle between the fluid and the fiber, or itssurface, is less than 90°, or when the fluid tends to spreadspontaneously across the surface of the fiber, both conditions arenormally co-existing. Conversely, a fiber or surface of the fiber isconsidered to be hydrophobic if the contact angle is greater than 90°and the fluid does not spread spontaneously across the surface of thefiber.

The terms “fiber” and “filament” are used interchangeably. The terms“nonwoven”, “nonwoven fabric” and “nonwoven web” are usedinterchangeably.

The term “electrolyte” means an ionic substance which increases theelectrical conductivity of water when dissolved in water.

Cellulosic Fibers

The term “individualized, crosslinked fibers”, refers to fibers thathave primarily intrafiber chemical crosslink bonds. That is, thecrosslink bonds are primarily between polymer (e.g. cellulose) moleculesof a single fiber, rather than between polymer molecules of separatefibers.

The term “cellulosic fiber” is a collective term for fibers made fromnatural cellulose, from regenerated cellulose or from cellulose esters.Cellulosic fibers from natural cellulose can be e.g. seed fibers or bastfibers. Regenerated cellulose can be made e.g. by dissolving andre-precipitating cellulose. Cellulosic fibers of diverse natural originare applicable to the invention. Digested fibers from softwood, hardwoodor cotton linters are preferably utilized. Fibers from Esparto grass,bagasse, kemp, flax, and other ligneous and cellulosic fiber sources mayalso be utilized as raw material in the invention. The fibers may besupplied in slurry, unsheeted or sheeted form. Fibers supplied as wetlap, dry lap or other sheeted form are preferably rendered intounsheeted form by mechanically disintegrating the sheet, preferablyprior to contacting the fibers with the crosslinking agent. Also,preferably the fibers are provided in a wet or moistened condition. Mostpreferably, the fibers are never-dried fibers. In the case of dry lap,it is advantageous to moisten the fibers prior to mechanicaldisintegration in order to minimize damage to the fibers. The optimumfiber source utilized in conjunction with this invention will dependupon the particular end use contemplated. Generally, pulp fibers made bychemical pulping processes are preferred. Completely bleached, partiallybleached and unbleached fibers are applicable. It may frequently bedesired to utilize bleached pulp for its superior brightness andconsumer appeal. Wood fibers that have been at least partially bleachedare preferred for use in the process of the present invention. Forproducts such as paper towels and absorbent pads for diapers, sanitarynapkins, catamenials, and other similar absorbent paper products, it isespecially preferred to utilize fibers from southern North Americasoftwood pulp due to their premium absorbency characteristic.

Crosslinking Agent

Suitable acidic crosslinking agents according to the invention includeagents having at least three acidic groups per molecule, wherein theacidic groups can react with hydroxyl groups of cellulosic fibers toform ester bonds. At least two of the acidic groups of one crosslinkingagent molecule may react with hydroxyl groups of at least two cellulosicfiber molecules. The reaction may occur between two cellulose moleculesof the same fiber to form intra-fiber ester bonds. One suitable exampleof an acidic crosslinking agent is a polymer having a plurality (i.e.three or more) of acidic functional groups. Acidic functional groups maybe for example carboxylic acid, sulfonic acid, or phosphoric acidgroups. In one embodiment, the acidic functional groups includecarboxylic acid groups. The polyacrylic acid polymers and copolymersdescribed above may be used as acidic crosslinking agents alone or incombination with other polycarboxylic acids such as, for example, citricacid.

The acidic cross-linking agent may be a homopolymer, obtainable from asingle type of monomer, wherein the monomer has at least one acidicgroup. The acidic cross-linking agent may also be a copolymer obtainablefrom at least two different types of monomers, wherein at least one typeof monomer has at least one acidic group and further types of monomersmay have no acidic groups. The acidic cross-linking agent may be derivedfrom natural or from synthetic sources. Suitable monomers containingacid groups include, for example, acrylic acid, methacrylic acid,crotonic acid, maleic acid, maleic acid monoesters. Acrylic acidpolymers, i.e. polymers obtainable by polymerizing acrylic acid or byco-polymerizing acrylic acid with at least one other monomer differentfrom acrylic acid, may also be suitable. Co-monomers that are notsubstituted with acid groups include, for example, acrylamide,methacrylamide, alkyl acrylamide, dialkyl acrylamide, alkylmethacrylamide, dialkyl methacrylamide, alkyl acrylate, alkylmethacrylate, vinylcaprolactone, vinylpyrrolidone, vinyl ester, vinylalcohol, wherein the alkyl groups of these monomers are C1 to C10 alkylgroups; or C1, C2, C3, or C4 alkyl groups. The alkyl groups may belinear or branched.

Acidic crosslinking agents include polyacrylic acid polymers, copolymersof acrylic acid, and mixtures thereof. Particularly suitable examples ofpolyacrylic acid crosslinking agents include copolymers of polyacrylicacid and maleic acid and the low molecular weight monoalkyl substitutedphosphinate and phosphonate copolymers described in U.S. Pat. No.5,256,746 to Blankenship, et al. These polymers may be especiallysuitable for crosslinking individualized cellulose fibers as describedherein and may exhibit non-negative effects on cellulose brightness whenused in a crosslinking process. Also, these polymeric acidiccrosslinking agents may exhibit a positive influence on the absorptioncapacity of the resulting crosslinked cellulosic fibers due to a lowerglass transition temperature when compared to monomeric acidiccrosslinking agents such as, e.g., citric acid. Polyacrylic acidpolymers may be made by polymerizing acrylic acid CH₂═CH—COOH to formthe repeating chain

—CH2-CH(COOM)—

wherein M is an alkali metal, ammonium or hydrogen. In the final,crosslinked cellulosic fibers, M may be hydrogen or at leastpredominantly hydrogen without metal ions or metal ions present only insuch minor amounts that they do not significantly reduce the cationreducing capacity of the material. Polymers of this type are available,for example, from the Rohm and Haas Company. The molecular weights ofexemplary copolymers may range from 500-40,000 or even from about 1,000to about 20,000. The weight ratio of acrylic acid to maleic acid mayrange from about 10:1 to about 1:1 or even from about 5:1 to 1.5:1. Aparticularly preferred copolymer may contain about 65% by weight acrylicacid and 35% by weight maleic acid. Another group of acrylic acidcopolymers that may be applicable to this invention are the lowmolecular weight monoalkyl substituted phosphinate and phosphonatecopolymers described in U.S. Pat. No. 5,256,746. These copolymers may beespecially suitable, since they provide fibers with high levels ofabsorbency, resiliency and brightness, and are generally safe andnon-irritating to human skin. These copolymers may be prepared withhypophosphorus acid and its salts (commonly sodium hypophosphite) and/orphosphorus acid as chain transfer agents. Molecular weights of thesetypes of copolymers may be below 20,000, below 3,000, and even betweenabout 1,000 and 2,000.

The molecular weight of the polymeric, acidic crosslinking agentssuitable for use in certain embodiments may be from 500 to 40,000 oreven from 1,000 to 20,000. In case of copolymers of acrylic acid, theweight ratio of acrylic acid to further monomers (e.g. maleic acid) canrange from 10:1 to 1:1, more preferably from 5:1 to 1.5:1.

Crosslinking of Cellulosic Fibers

In certain embodiments, the crosslinked cellulosic fibers and the methodof making them may be those described in PCT Pub. No. WO 95/34710 toHerron, et al. In certain embodiments, the individualized, crosslinkedcellulosic fibers may have an effective amount of the polymeric acidcrosslinking agent reacted with the fibers in the form of intra-fibercrosslink bonds. As used herein, “effective amount of crosslinkingagent” refers to an amount of crosslinking agent sufficient to providean improvement in at least one significant absorbency property of thefibers themselves and/or absorbent structures containing theindividualized, crosslinked fibers, relative to conventional,uncrosslinked fibers. One example of a significant absorbency propertyis drip capacity, which is a combined measurement of an absorbentstructure's fluid absorbent capacity and fluid absorbency rate asdescribed in WO 95/34710. The crosslinked cellulosic fibers may have,for example, from 1 wt. % to 50 wt. % or from 5 wt. % to 30 wt. %, orfrom 10 wt. % to 20 wt. % crosslinking agent, calculated on a dry fiberbasis, reacted with the fibers. The crosslinking agent may be contactedwith the fibers in a liquid medium, under such conditions that thecrosslinking agent penetrates into the interior of the individual fiberstructures. However, other methods of crosslinking agent treatment,including spraying or spray and press, dip and press, etc., of thefibers while in individualized, fluffed form, or sheeted form are alsocontemplated herein.

Once the fibers are treated with crosslinking agent (and catalyst if oneis used), the crosslinking agent may be reacted with the fibers in thesubstantial absence of inter-fiber bonds, i.e., while inter-fibercontact is maintained at a low degree of occurrence relative tounfluffed pulp fibers, or the fibers are submerged in a solution thatdoes not facilitate the formation of inter-fiber bonding. This mayresult in the formation of crosslink bonds which are intra-fiber innature. Under these conditions, the crosslinking agent may reactpredominantly to form crosslink bonds between hydroxyl groups of asingle cellulose chain or between hydroxyl groups of proximately locatedcellulose chains of a single cellulosic fiber. Although not presented orintended to limit the scope of the invention, it is believed that theacid groups on the acidic polymeric crosslinking agent react with thehydroxyl groups of the cellulose to form ester bonds. The formation ofester bonds, believed to be the desirable bond type providing stablecrosslink bonds, is favored under acidic reaction conditions. Therefore,acidic crosslinking conditions, i.e., pH ranges of from about 1.5 toabout 5, may be present in certain embodiments. The fibers may bemechanically defibrated into a low density, individualized, fibrous formknown as “fluff” prior to reaction of the crosslinking agent with thefibers. Mechanical defibration may be performed by a variety of methodswhich are presently known in the art.

The crosslinked cellulosic fibers may have unique combinations ofstiffness and resiliency, which may allow absorbent structures made fromthe fibers to maintain high levels of absorptivity, and exhibit highlevels of resiliency and an expansionary responsiveness to wetting of adry, compressed absorbent structure. In addition to having the levels ofcrosslinking within the stated ranges, the crosslinked fibers may becharacterized as having water retention values (“WRVs”) of up to 100,e.g. less than about 60, between about 25 to about 50, or even betweenabout 30 and about 45. The WRV of a particular fiber may be indicativeof the level of crosslinking for a particular crosslinking chemistry andmethod. A procedure for measuring WRV is given in WO 95/34710.

Neutralization

Not all acidic groups of the acidic crosslinking polymer undergo esterbond reactions with cellulosic hydroxyl groups, i.e. non-reacted, freeacid groups remain in the individualized, crosslinked cellulosic fibers.In certain embodiments, these remaining acid groups may be at leastpartially neutralized by one or more basic polymers. In certainembodiments the at least partial neutralization may occur when thefibers are wetted. The polymers may comprise at least four, or eveneight or more monomeric units. The basic polymers may have a plurality(e.g. three or more) of base groups. Suitable nonlimiting examples ofbase groups include primary, secondary, tertiary amine groups orquaternary ammonium hydroxide groups. Examples of polymers suitable foruse herein include, without limitation, those polymers prepared frompolymerizable monomers comprising base groups or groups that can beconverted to base groups after polymerization. Thus, such monomers mayinclude those which contain primary, secondary, and/or tertiary aminegroups, or the corresponding phosphines or quaternary ammonium groups.The amount of the basic polymer can be such that the degree ofneutralization of the acid groups on the crosslinked cellulosic fiber isfrom 50% to 100%, from 70% to 100%, from 90% to 100%, or even 100%. Incertain embodiments, the degree of neutralization may be controlled bythe appropriate selection of the amount of basic polymer. The amount ofbasic polymer may be, for example from 0.5% to 65 wt. %, from 1% to 50wt. %, or even from 2% to 40 wt. %, based on the total amount of thematerial. The basic polymer may be used as neutralizing agent alone orin combination with other, water soluble, organic or inorganicnon-polymeric bases. In certain embodiments, base groups of the basicpolymer may be amine groups such as, for example, primary, secondary, ortertiary amine groups.

The basic polymer may be a homopolymer, obtainable from a single type ofmonomer, wherein the monomer has at least one base group. The basicpolymer may be a copolymer obtainable from at least two different typesof monomers, wherein at least one type of monomer has at least one basegroup and further types of monomers have no base groups. The basicpolymer may be derived from natural (e.g. comprising nucleobases) orsynthetic sources. The basic polymers may also be random, graft, orblock copolymers, and may have linear or branched architectures.Suitable monomers containing base groups include, but are not limitedto, vinylamine, allylamine, diallylamine, ethyleneimine(aziridine),4-aminobutene, alkyl oxazolines, 5-aminopentene, carbodiimides,formaldazine, melamine, dialkylaminoalkyl acrylate, dialkylaminoalkylmethacrylate, dialkylaminoalkyl acrylamide, dialkylaminoalkylmethacrylamide, vinylguanidine, allylguanidine and the like, as well astheir secondary or tertiary amine derivatives, e.g. N-monoalkyl- orN,N-di-lkyl compounds with from 1 to 4 carbon atoms.

Basic polymers derived from natural sources include, for example,diethyl amino ethyl (“DEAE”) cellulose, polyethyleneimine (“PEI”)cellulose, amino ethyl cellulose, triethyl amino ethyl cellulose,guanidoethyl cellulose, paraminobenzyl cellulose, ECTEOLA cellulose(triethanolamine coupled to cellulose through glyceryl and polyglycerylchains), benzoylated DEAE cellulose, and benzoylated-naphthoylated DEAEcellulose prepared by conventional techniques. DEAE cellulose, forexample, can be prepared by treating cellulose with a solution of2-(diethylamino) ethyl chloride.

Synthetic basic polymers include, for example, poly(vinylamine),poly(allylamine), polyethylenimine, poly(dialkylaminoalkyl acrylamide),poly(dialkylaminoalkyl methacrylamide), poly(dialkylaminoalkylacrylate), poly(dialkylaminoalkyl methacrylate) or polymeric resinscontaining quaternary ammonium hydroxide groups. Basic polymers may beprepared from at least one monomer having the general structure:

R₁HC═CR₂—C(═O)—NH—Y—NR₃R₄; or  (I)

R₁HC═CR₂—C(═O)—O—Y—NR₃R₄,  (II)

wherein R₁ and R₂, independently, are selected from the group consistingof hydrogen and methyl, Y is a divalent organic radical that can belinear or branched having 1 to 8 carbon atoms, and R₃ and R₄,independently, are alkyl radicals having 1 to 4 carbon atoms. R₁ may behydrogen, R₂ may be hydrogen or methyl, Y may have 2 or 3 carbon atoms,and R₃ and R₄ may be the same and have 1 or 2 carbon atoms. Furthernonlimiting examples of basic polymers are poly(vinylguanidine) andpoly(allylguanidine).

In certain embodiments, basic polymers may include a variety ofwater-insoluble, but water-swellable polymers. The basic polymers aretypically lightly crosslinked polymers that contain a multiplicity ofbase functional groups, such as primary, secondary and/or tertiaryamines; or the corresponding phosphines. The polymers may be renderedwater-insoluble, but water-swellable, by a relatively low degree ofcrosslinking. This may be achieved by including the appropriate amountof a suitable crosslinking monomer during the polymerization reaction.Examples of crosslinking monomers include N,N′-methylenebisacrylamide,ethylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate,triallylamine, diaziridine compounds, and the like. Alternatively, thepolymers can be crosslinked after polymerization by reaction with asuitable crosslinking agent such as di- or poly-halogenated compoundsand/or di- or poly-epoxy compounds. Examples include diiodopropane,dichloropropane, ethylene glycol diglycidyl ether, and the like.

Another exemplary basic polymer is crosslinked divinylbenzene/styrenecopolymer containing quaternary ammonium groups in its hydroxide form,e.g. AMBERLYST A26 OH of Rohm and Haas, or any other kind of weak baseor strong base anion exchange polymer.

The basic polymer may be of one type (i.e. homogeneous), however,mixtures of base polymers may be used in certain embodiments. Forexample, mixtures of polyethylenimine (which may be crosslinked) andpolyallylamine (which may be crosslinked) be suitable for use in certainembodiments.

Before neutralizing acidic groups of the crosslinked cellulosic fiber,the basic polymer may be from about 50% to about 100%, about 80% toabout 100%, or even from about 90% to about 100%, in the un-neutralizedbase form. In order to potentially improve the electrolyte concentrationreducing capacity of the liquid acquisition material, it may bedesirable to provide a basic polymer that exhibits a relatively highamount of amine groups per gram of dry polymer. In certain embodiments,the amine group density of the basic polymer component may be at least 4milliequivalents per gram (“meq/g”), at least 6 meq/g, at least 10meq/g, at least about 15 meq/g, and even at least about 20 meq/g.

In order to further increase the potential improvement of theelectrolyte concentration reduction capacity of the liquid acquisitionmaterial, it may be desirable to provide a material comprisingapproximately equal equivalents of acid groups and base groups. However,it may be desirable to have somewhat more equivalents of acid groups orof base groups, e.g., to compensate for differences in pK, to compensatefor differences in neutralization, to alter the pH of (for example toacidify) the liquid to be acquired, etc. The approximate electrolyteconcentration reducing capacity of the liquid acquisition material ofthe invention can be calculated from the acid and base strength of theconstituent acidic crosslinked cellulosic fiber material and the basicpolymer.

In certain embodiments, the electrolyte concentration reducing capacityof the liquid acquisition material of the invention may be at least 0.05meq/g, at least 0.1 meq/g, or even at least about 0.3 meq/g.

Disposable Absorbent Article

In certain embodiments, a disposable absorbent article comprising theabove described material for acquisition of liquids may be provided. Forexample it may be desirable to provide a disposable absorbent articlecomprising a liquid pervious topsheet, a liquid impervious backsheet, aliquid storing absorbent core layer comprising super-absorbent materialpositioned between said topsheet and said backsheet and a liquidacquiring and distributing layer comprising the material for acquisitionof liquids described hereinabove. At least part of the liquidacquisition material may be disposed, for example, between the topsheetand absorbent core layer of the disposable absorbent article such thatthe liquid to be absorbed (e.g. urine) is contacted with the liquidacquisition material before it is contacted with the core layer.

FIG. 1 shows a plan view of a diaper 20. The diaper 20 is shown in itsflat out, uncontracted state (i.e., without elastic inducedcontraction). Portions are cut away to more clearly show the underlyingstructure of the diaper 20. The portion of the diaper 20 that contacts awearer is facing the viewer. The diaper 20 may include a liquid pervioustopsheet 24 that faces a wearer when the diaper is worn as intended. Thechassis 22 of the diaper 20 in FIG. 1 may be configured as the main bodyof the diaper 20. The chassis 22 may include a liquid imperviousbacksheet 26. The chassis may also include most or all of the absorbentcore 28 encased between the topsheet 24 and the backsheet 26. Thechassis may further include side panels 30, leg cuffs 32 and a waistfeature 34. The leg cuffs and the waist feature typically compriseelastic members 33. One end portion of the diaper 20 may be configuredas the front waist region 36 of the diaper 20. The opposite end portionmay be configured as the rear waist region 38 of the diaper 20. Anintermediate portion of the diaper 20 may be configured as the crotchregion 37, which extends longitudinally between the front and rear waistregions 36 and 38. The crotch region 37 is the portion of the diaper 20that is generally positioned between the wearer's legs when the diaper20 is worn as intended. The diaper 20 may have a longitudinal axis 100and a transverse axis 110. The periphery of the diaper 20 is defined bythe outer edges of the diaper 20 in which the longitudinal edges 44 rungenerally parallel to the longitudinal axis 100 of the diaper 20 and theend edges 46 run generally parallel to the transverse axis 110 of thediaper 20.

For unitary absorbent articles, the chassis 22 comprises the mainstructure of the diaper with other features added to form the compositediaper structure. The topsheet 24, the backsheet 26, and the absorbentcore 28 may be assembled in a variety of well-known configurations.Specific diaper configurations are described generally in U.S. Pat. No.5,569,234 entitled “Disposable Pull-On Pant” issued to Buell et al. onOct. 29, 1996; and U.S. Pat. No. 6,004,306 entitled “Absorbent ArticleWith Multi-Directional Extensible Side Panels” issued to Robles et al.on Dec. 21, 1999.

The topsheet 24 in FIG. 1 may be fully or partially elasticized or maybe foreshortened to provide a void space between the topsheet 24 and theabsorbent core 28. Exemplary structures including elasticized orforeshortened topsheets are described in more detail in U.S. Pat. No.5,037,416 entitled “Disposable Absorbent Article Having ElasticallyExtensible Topsheet” issued to Allen et al. on Aug. 6, 1991; and U.S.Pat. No. 5,269,775 entitled “Trisection Topsheets for DisposableAbsorbent Articles and Disposable Absorbent Articles Having SuchTrisection Topsheets” issued to Freeland et al. on Dec. 14, 1993.

The backsheet 26 may be joined with the topsheet 24. The backsheet 26may be configured to prevent exudates absorbed by the absorbent core 28and contained within the diaper 20 from soiling other external articlesthat may contact the diaper 20, such as bed sheets and undergarments.Often, the backsheet 26 is substantially impervious to liquids (e.g.,urine) and comprises a laminate of a nonwoven and a thin plastic filmsuch as a thermoplastic film having a thickness of about 0.012 mm (0.5mil) to about 0.051 mm (2.0 mils). Suitable backsheet films includethose manufactured by Tredegar Industries Inc. of Terre Haute, Ind. andsold under the trade names X15306, X10962, and X10964. Other suitablebacksheet materials may include breathable materials that permit vaporsto escape from the diaper 20 while still preventing exudates frompassing through the backsheet 26. Exemplary breathable materials includematerials such as woven webs, nonwoven webs, composite materials such asfilm-coated nonwoven webs, and microporous films such as manufactured byMitsui Toatsu Co., of Japan under the designation ESPOIR NO and by EXXONChemical Co., of Bay City, Tex., under the designation EXXAIRE.

The absorbent core 28 in FIG. 1 generally is disposed between thetopsheet 24 and the backsheet 26. The absorbent core 28 may comprise anyabsorbent material that is generally compressible, conformable,non-irritating to the wearer's skin, and capable of absorbing andretaining liquids such as urine and other certain body exudates. Theabsorbent core 28 may be manufactured in a wide variety of sizes andshapes (e.g., rectangular, hourglass, “T”-shaped, asymmetric, etc.) andmay comprise a wide variety of liquid-absorbent materials commonly usedin disposable diapers and other absorbent articles such as comminutedwood pulp, which is generally referred to as air felt. Examples of othersuitable absorbent materials include creped cellulose wadding; meltblown polymers, including co-form; chemically stiffened, modified orcross-linked cellulosic fibers; tissue, including tissue wraps andtissue laminates, absorbent foams, absorbent sponges, superabsorbentpolymers, absorbent gelling materials, or any other known absorbentmaterial or combinations of materials. The absorbent core may furthercomprise minor amounts (typically less than 10%) of non-liquid absorbentmaterials, such as adhesives, waxes, oils and the like. In oneembodiment, the core comprises superabsorbent polymers and is air feltfree.

Exemplary absorbent structures for use as the absorbent assemblies aredescribed in U.S. Pat. No. 4,834,735, entitled “High Density AbsorbentMembers Having Lower Density and Lower Basis Weight Acquisition Zones”,issued to Alemany et al. on May 30, 1989; and U.S. Pat. No. 5,650,222entitled “Absorbent Foam Materials For Aqueous Fluids Made From highInternal Phase Emulsions Having Very High Water-To-Oil Ratios” issued toDesMarais et al. on Jul. 22, 1997.

The diaper 20 may also include such other features as are known in theart including front and rear ear panels, waist cap features, elasticsand the like to provide better fit, containment and aestheticcharacteristics. Such additional features are well known in the art andare described in U.S. Pat. No. 3,860,003 entitled “Contractable sideportions for disposable diaper” issued to Buell et al. on Jan. 14, 1975and U.S. Pat. No. 5,151,092 entitled “Absorbent article with dynamicelastic waist feature having a predisposed resilient flexural hinge”issued to Buell et al. on Sep. 29, 1992.

In order to keep the diaper 20 in place about the wearer, the waistregions 36 and 38 may include a fastening system comprising fasteningmembers 40 attached to the rear waist region 38. In one embodiment thefastening system further comprises a landing zone 42 attached to thefront waist region 36. The fastening member is attached to the frontwaist region 36, often to the landing zone 42, to form leg openings andan article waist. Diapers 20 according to the present invention may beprovided with a re-closable fastening system or may alternatively beprovided in the form of pant-type diapers. The fastening system and anycomponent thereof may include any material suitable for such a use,including but not limited to plastics, films, foams, nonwoven webs,woven webs, paper, laminates, fiber reinforced plastics and the like, orcombinations thereof. In some embodiments, the materials making up thefastening device are flexible. The flexibility is designed to allow thefastening system to conform to the shape of the body and thus, reducesthe likelihood that the fastening system will irritate or injure thewearer's skin.

FIG. 2 shows a cross-sectional view of FIG. 1 taken in the transverseaxis 110. Starting from the wearer facing side the diaper comprises thetopsheet 24, the components of the absorbent core 28, and the backsheet26. An acquisition system 50 is disposed between the topsheet 24 and thebacksheet 26, preferably between the topsheet 24 and the absorbent core28. The acquisition system 50 may comprise an upper acquisition layer 52facing towards the wearer and a lower acquisition layer 54.

In certain embodiments, the liquid acquisition system 50 may comprisethe liquid acquisition material. The liquid acquisition material may bedisposed either in the upper acquisition layer 52 or in the loweracquisition layer 54 or in both. In certain embodiments the upperacquisition layer 52 comprises a nonwoven fabric and the loweracquisition layer 54 comprises the liquid acquisition material. Incertain embodiments, both the upper and lower acquisition layerscomprise the liquid acquisition material. When the acquisition system 50comprises a non-woven fabric, the non-woven fabric may be hydrophilic.In certain embodiments, the acquisition layer may be in direct contactwith the absorbent core 28.

The storage layer 60 may be wrapped by a core wrap material. In certainembodiments, the core wrap material may comprise a top layer 56 and abottom layer 58. The top layer 56 and the bottom layer 58 may include anon-woven material. One useful nonwoven material is a so-called SMSmaterial, which is commonly known in the art as a three-layer materialcomprising a spunbond layer, a meltblown layer, and another spunbondlayer. The top layer 56 and the bottom layer 58 may be provided from twoor more separate sheets of materials or they may be provided from aunitary sheet of material. Such a unitary sheet of material may bewrapped around the storage layer 60, e.g. in a C-fold. The top layer 56and the bottom layer 58 may also be joined to each other, for examplealong their periphery. In certain embodiments, both layers may be joinedalong their longitudinal and/or transversal peripheries. The joining canbe achieved my multiple means well known in the art, e.g. by adhesivemeans, using a continuous or a discontinuous pattern, for example alinear or curvilinear pattern. The storage layer 60 may comprise fibrousmaterials, mixed with superabsorbent polymers and/or absorbent gellingmaterials. Other materials described above as suitable for the absorbentcore 28 may also be included. In one embodiment, the storage layer 60has reduced amounts of fibrous materials or is free of fibrous materialsand the concentration of superabsorbent polymer and/or absorbent gellingmaterials in the storage layer 60 is at least 40 wt. %, at least 60 wt.% or at least 90 wt. %, based on the total amount of absorbent materialin the storage layer 60.

Method of Use

In certain embodiments, a method of reducing the electrolyteconcentration of an electrolyte containing aqueous medium is disclosed.The method may comprise contacting an aqueous medium which containselectrolytes with one or more examples of the liquid acquisitionmaterial described above.

Method of Making

In certain embodiments, a process for making one or more examples of theliquid acquisition material described above may comprise a) providing acellulosic based fiber, b) impregnating the fiber with the at least oneacidic crosslinking agent and with the at least one basic substance, andc) heating the resulting mixture to temperatures of at least the boilingpoint of water. Optionally, the resulting crosslinked fiber mixture maybe baled, as is commonly known in the art.

EXAMPLES

The materials illustrated in the following examples illustrate specificembodiments of the present invention, but are not intended to belimiting thereof. Other modifications can be undertaken by the skilledartisan without departing from the spirit and scope of this invention.

All exemplified amounts are listed as weight percents and exclude minormaterials such as diluents, preservatives, colour solutions, imageryingredients, botanicals, and so forth, unless otherwise specified. If atrade name is mentioned as ingredient and the respective product isitself a mixture (e.g. a solution, emulsion, dispersion etc.), then theexemplified amount relates to this mixture, unless otherwise specified.

In the following examples, 4 g patches, made of individualized,crosslinked cellulosic fibers are used. The fiber material is madeaccording to Example II of WO 95/34710 except that the crosslinkedfibers contain 8 wt. % polyacrylic acid (i.e. 0.32 g of polyacrylic acidare in a 4 g fiber patch), calculated on a dry fiber weight basis,reacted with the fibers in the form of intrafiber crosslink bonds.

Polyallylamine (PAAm) is commercially available as 20 wt. % aqueoussolutions. Crosslinked quaternary ammonium hydroxidedivinylbenzene/styrene copolymer is commercially available (AmberlystA-260H) as water containing spherical resin beads. Both chemicals areused without further purification. The density of the 20 wt. % aqueoussolution of polyallylamine is estimated to be ˜1 g/ml.

Different saline (NaCl) solutions from 0.8 to 0.9 wt. % are prepared andtheir conductivity measured. Within this concentration range, there isan almost linear relationship between conductivity and NaClconcentration. This relationship is used as reference to determine theNaCl concentration of the test samples by measuring the conductivity ofthe test samples.

The conductivity of 0.9 wt. % saline solutions before and after contactwith the test material is measured, corrected for the conductivityeffect of the basic material (polyallylamine or Amberlyst A-26 OH,respectively) and corrected for the dilution effect of water. Thedecrease in NaCl concentration (desalting effect) of the test samples isdetermined from the conductivity/concentration relationship.

Conductivity Measurement Method Equipment: Conductivity Meter: WTW LF320 Stirrer and Hot Plate: IKA RH-KT/C Eppendorf Pipette

The fiber patch is washed several times with distilled water and driedat 50° C. to remove all extractable components that might influence theconductivity measurements.

All conductivity measurements are carried out at room temperature or at37° C. but the conductivity meter is used in “auto-correlation” mode sothat the given conductivities are automatically correlated to 25° C. 4 gPAA-crosslinked cellulosic fiber pads are merged into 200 ml 0.9 wt. %saline (NaCl) solution and after 5 minutes of stirring the conductivityis measured. This conductivity is set as the base for the series ofmeasurements and for the following calculations.

Example 1

4 g washed and dried polyacrylic acid crosslinked cellulosic fiber padis merged into 200 ml 0.9 w % saline solution at room temperature, andafter 5 min of manually stirring the conductivity is measured.Polyallylamine (PAAm) is added. The mixture is stirred and theconductivity is observed. From the conductivity change the concentrationchange of the NaCl solution is calculated and corrected for dilutioneffects.

Amount PAAm added 0.28 g Difference in conductivity −0.95 mS/cmEffective NaCl reduction −0.051 wt. %

Example 2

4 g washed and dried polyacrylic acid crosslinked cellulosic fiber padis merged into 200 ml 0.9 w % saline solution at room temperature andafter 5 min of manually stirring the conductivity is measured. AmberlystA-26(OH) is added. The mixture is stirred and the conductivity isobserved. From the conductivity change the concentration change of theNaCl solution is calculated and corrected for dilution effects.

Amount Amberlyst A-26(OH) added 1.8 g Difference in conductivity −1.08mS/cm Effective NaCl reduction −0.065 wt. %

Example 3

4 g washed and dried polyacrylic acid crosslinked cellulosic fiber isstrongly mixed for 10 minutes with 1.5 g Amberlyst A-26(OH) and thengently compressed with a pistil and 0.3 psi. A 0.9 wt. % saline solutionis tempered to 37° C. and 100 ml of the saline solution is poured in onegush on top the cellulosic fiber pad/A-26(OH) mixture to allow theliquid to flow through the pad and the solution was collectedafterwards. This is repeated two more times and the conductivity beforeand afterwards as well as the amount of liquid is measured.

From the conductivity change the amount of absorbed NaCl is determinedto be 1.249 mmol after 3 gushes of 100 ml (280 ml recovered).

The dimensions and values disclosed herein are not to be understood asbeing strictly limited to the exact numerical values recited. Instead,unless otherwise specified, each such dimension is intended to mean boththe recited value and a functionally equivalent range surrounding thatvalue. For example, a dimension disclosed as “40 mm,” is intended tomean “about 40 mm”.

All documents cited in the Detailed Description of the Invention are, inrelevant part, incorporated herein by reference; the citation of anydocument is not to be construed as an admission that it is prior artwith respect to the present invention. To the extent that any meaning ordefinition of the term in a document incorporated by reference, themeaning or definition assigned to the term in this written documentshall govern.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

1. A material for the acquisition of liquids, said material comprising:a. at least one basic polymer; and b. individualized, crosslinkedcellulosic fibers including an effective amount of at least one acidiccrosslinking agent, said crosslinking agent reacted with said fibers inintra-fiber crosslink ester bond form, wherein said acidic crosslinkingagent is a polymer comprising a plurality of acidic functional groups.2. The material according to claim 1, wherein the polymeric acidcrosslinking agent is selected from the group consisting of homopolymersof acrylic acid, copolymers of acrylic acid, and mixtures thereof. 3.The material according to claim 2, wherein the polymeric acidcrosslinking agent is poly(acrylic acid).
 4. The material according toclaim 1, wherein the at least one basic polymer is selected from thegroup consisting of polymers containing a plurality of amine groups andpolymers containing a plurality of quaternary ammonium hydroxide groups.5. The material according to claim 1, wherein said fibers have from 1weight % to 10 weight % of said crosslinking agent, calculated on a dryfiber weight basis, reacted therewith in the form of intrafibercrosslink ester bonds, and wherein said crosslinked fibers have a waterretention value of up to
 100. 6. The material according to claim 1,wherein said basic polymer is made from at least one monomer containingat least one primary, secondary or tertiary amine group, said at leastone monomer being selected from the group consisting of vinylamine,allylamine, diallylamine, ethyleneimine, 4-aminobutene, alkyloxazolines, 5-aminopentene, melamine, dialkylaminoalkyl acrylate,dialkylaminoalkyl methacrylate, dialkylaminoalkyl acrylamide,dialkylaminoalkyl methacrylamide, vinylguanidine and allylguanidine,preferably poly(allylamine), poly(ethylenimine), poly(vinylamine). 7.The material according to claim 1, wherein said material contains from 1weight % to 50 weight % of said at least one polymer containing basegroups, based on the total amount of the material.
 8. The materialaccording to claim 1, wherein said material exhibits an electrolyteconcentration reducing capacity of at least 0.05 meq/g.
 9. A materialfor acquisition of liquids, said material comprising: a. individualized,crosslinked cellulosic fibers including an effective amount of at leastone acidic crosslinking agent reacted with said fibers in intra-fibercrosslink ester bond form, said acidic crosslinking agent being apolymer comprising a plurality of acidic functional groups; and b. atleast one conductivity reducing substance.
 10. The material according toclaim 9, wherein the material reduces the electrical conductivity of a0.9 wt. % NaCl solution by at least 0.3 mS/cm according to theConductivity Measurement Method when said NaCl solution is contactedwith said material.
 11. A process for making a material for acquisitionof liquids, said material comprising individualized, crosslinkedcellulosic fibers including an effective amount of at least one acidiccrosslinking agent reacted with said fibers in intra-fiber crosslinkester bond form, said acidic crosslinking agent being a polymercomprising a plurality of acidic functional groups; and at least onebasic polymer or at least one conductivity reducing substance, saidmethod comprising: a. providing a cellulosic based fiber, b.impregnating the fiber with the at least one acidic crosslinking agentand with the at least one basic polymer or at least one conductivityreducing substance, and c. heating the resulting mixture to temperaturesof at least the boiling point of water.